The present invention relates in general to digital-to-analog (D/A) converters and in particular to a device for controlling the output voltage range of a D/A converter of the type wherein the converter output voltage range is proportional to the magnitude of an applied bias current.
D/A converters convert digitally encoded signals to analog voltage signals of proportionate magnitude. Typically a digital logic circuit, generating digital input data and signals controlling the operation of a D/A converter, uses the converter to generate various output voltages for reference or control purposes.
The output voltage range of a D/A converter is typically a function of an applied bias signal (current or voltage), the D/A converter output voltage being proportional to the product of the magnitudes of the digital input and the applied bias signal. The resolution of a D/A converter within a range determined by the applied bias signal depends on the input bit-handling capability of the converter. For example a converter designed for a four-bit digital input can produce 16 different output voltage levels while an eight-bit converter can produce 256 different output voltages. Higher resolution converters are usually more complex and expensive than low resolution converters and require the use of more complex and expensive external control circuitry. In some applications it would be advantageous if external control circuitry capable of controlling, for instance, a four-bit data bus, could cause a four-bit D/A converter to produce a large number of different output voltages.
In the prior art, a larger number of output voltage levels has been obtained from a low resolution converter by taking advantage of the dependency of D/A converter range on the applied bias signal. If the bias signal of a first D/A converter is not constant but is dependent on the output of a second D/A converter, then the number of different output voltage levels which can be generated by the first D/A converter greatly increases. For instance, a four-bit D/A converter biased with the output of a second four-bit D/A converter would operate over 15 different output ranges, not counting the null range resulting from a zero bias current. Though some of the voltage levels in one range may overlap with voltage levels in another range, the total number of possible distinct output voltage levels obtainable is increased. However, use of second D/A converter is expensive.